Ecology of Hup+ Rhizobium strains of cow pea miscellany: native frequency and competence

Rhizobium strains nodulating summer legumes cow pea [Vigna unguiculata (L.)], green gram [V. radiata (L.) (Wilczek)], black gram [V. mungo (L.) (Hepper)] and cluster bean [Cyamopsis tetragonoloba (L.) (Taub)] and a winter legume chick pea [Cicer arietinum (L.)] were surveyed in the Northern Plains of India and screened for hydrogenase activity to determine distribution of Hup character in the native ecosystem. It was observed that 56% of the Rhizobium strains of summer legumes were Hup⁺ whereas that of the winter legume, chick pea, were all Hup^-. Ex planta acetylene reduction activity was observed in most of the Hup⁺ but not in the Hup^- strains of any of the host species. In summer legume, mixed inoculation of Hup⁺ and Hup^- strains, under sterilized as well as unsterilized soil conditions, showed that the host species were predominantly nodulated with Hup⁺ strain.     

Uptake hydrogenase in Rhizobium and nodule leghemoglobin in cow pea miscellany hosts

Two effective strains of green gram rhizobia S24 (slow growing and Hup⁺) and M11 (fast growing and Hup^-) were tested for leghemoglobin production in nodules and effectivity on six species of cow pea miscellany hosts. Both strains nodulate green gram [Vigna radiata (L.) (Wilczek)], black gram [Vigna mungo (L.) (Hepper)], cow pea [Vigna unguiqulata (L.)], moth bean [Vigna aconitifolia (Jacq.) (Marechel)], Cluster bean [Cyamopsis tetragonoloba (L.) (Taub.)] and pigeon pea [Cajanus cajan (L.)]. In all these hosts, nodules formed by strain M11 contained 1.5 to 2 times more leghemoglobin than the nodules formed by strain S24. Gel electrophoresis of nodule contents of different host species showed a high concentration of a fast-moving ferricoxy leghemoglobin in the nodules of plants inoculated with strain M11 as compared to that of strain S24. Strain M11, however, was relatively less effective than strain S24 on black gram, cow pea and moth bean and was at par with the later on green gram, cluster bean and pigeon pea. Hydrogen recycling ability of the strain S24 was observed in nodules of all the host species. The effective functioning of strain S24 at low levels of leghemoglobin suggests an involvement of recycling hydrogenase in maintaining an appropriate oxidation-reduction potential in nodules.Abbreviations Lb Leghemoglobin - Cvr cultivar     

Zinc nutrition and leaf carbonic anhydrase activity ofEucalyptus maculata seedlings andTrifolium subterraneum

The competitive ability of eight strains ofBradyrhizobium on Vigna was examined. It was found that strains S24, M10, and M11 occupied a greater percent of nodules when introduced as mixed inoculum of two strains. Growth rate of strains did not affect competitive ability of the strains. Two hydrogen-uptake positive (Hup⁺) strains, S24 and M10, were found to be good competitors while another Hup⁺ strain GR4 was not so. Influence of the host in competition was observed in the case of strain GR4.     

Host Plant Cultivar Effects on Hydrogen Evolution by Rhizobium leguminosarum

The effect of host plant cultivar on H₂ evolution by root nodules was examined in symbioses between Pisum sativum L. and selected strains of Rhizobium leguminosarum. Hydrogen evolution from root nodules containing Rhizobium represents the sum of H₂ produced by the nitrogenase enzyme complex and H₂ oxidized by any uptake hydrogenase present in those bacterial cells. Relative efficiency (RE) calculated as RE = 1 − (H₂ evolved in air/C₂ H₂ reduced) did not vary significantly among `Feltham First,' `Alaska,' and `JI1205' peas inoculated with R. leguminosarum strain 300, which lacks uptake hydrogenase activity (Hup⁻). That observation suggests that the three host cultivars had no effect on H₂ production by nitrogenase. However, RE of strain 128C53 was significantly (P ≤ 0.05) greater in symbiosis with cultivar JI1205 than in root nodules of Feltham First. At a similar rate of C₂H₂ reduction on a whole-plant basis, nearly 24 times more H₂ was evolved from the Feltham First/128C53 symbiosis than from the JI1205/128C53 association. Root nodules from the Alaska/128C53 symbiosis had an intermediate RE over the entire study period, which extended from 21 to 36 days after planting. Direct assays of uptake hydrogenase by two methods showed significant (P ≤ 0.05) host cultivar effects on H₂ uptake capacity of both strain 128C53 and the genetically related strain 3960. The ³H₂ incorporation assay showed that strains 128C53 and 3960 in symbiosis with Feltham First had about 10% of the uptake hydrogenase activity measured in root nodules of Alaska or JI1205. These data are the first demonstration of significant host plant effects on rhizobial uptake hydrogenase in a single plant species.     

Comparative response ofPisum sativum nodulated with indigenous soilRhizobium populations and/or co-inoculated with aRhizobium leguminosarum strain

No significant differences in the acetylene-reducing activity and evolution of H₂ and CO₂ nodulated roots ofPisum sativum inoculated with soilRhizobium populations from two soils with different acidities (Ruzyně soil 7.6; Lukavec soil 4.9) were observed.Rhizobium population from Lukavec soil formed nodules, exhibiting a higher H₂ evolution. Co-inoculation with the Hup⁺ strain 128C30 (7×10⁷ cells per seedling) eliminated, to some extent, the effect of soil populations on physiological activity. Translated by Č. Novotny     

Hydrogen production and uptake by pea nodules as affected by strains of Rhizobium leguminosarum

Hydrogen evolution from root nodules has been reported to make N₂ fixation by some legume-Rhizobium symbiotic systems inefficient. We have surveyed the extent of H₂ evolution and estimated relative efficiencies of nodules of Austrian winter peas formed by 15 strains of R. leguminosarum. Their rates of H₂ evolution in air were about 30% of the rates of H₂ evolution under an atmosphere in which N₂ was replaced by Ar. Relative efficiency values based on C₂H₂ reduction rates ranged from 0.55 to 0.80. With some of the strains, hydrogenase activities were demonstrated in intact nodules and in bacteroids, but the levels of activity were insufficient to recycle all the H₂ evolved by the nitrogenase system. In both intact nodules and bacteroids the hydrogenase is less sensitive to O₂ damage than the nitrogenase system, so H₂ uptake capacity was observed in intact nodules by suppressing the nitrogenase-dependent H₂ evolution with an atmosphere containing a high O₂ concentration, and in bacteroids by using aerobically prepared bacteroid suspensions. The hydrogenase activity of both was dependent on O₂ consumption. A K _mfor H₂ of near 4 M was determined in suspension of bacteroids from nodules formed by strains 128C53 and 128C56.     

Hydrogen emission from nodulated soybeans [Glycine max (L.) Merr.] and consequences for the productivity of a subsequent maize (Zea mays L.) crop

Hydrogen (H₂) is a by-product of the symbiotic nitrogen fixation (N₂ fixation) between legumes and root-nodule bacteria (rhizobia). Some rhizobial strains have an uptake hydrogenase enzyme (commonly referred to as Hup⁺) that recycles H₂ within the nodules. Other rhizobia, described as Hup^?, do not have the enzyme and the H₂ produced diffuses from the nodules into the soil where it is consumed by microorganisms. The effect of this phenomenon on the soil biota and on the soil itself, and consequent stimulation of plant growth, has been demonstrated previously. Soybeans [Glycine max (L.) Merr.] cv. Leichhardt, inoculated with either a Hup⁺ strain (CB1809) or one of two Hup^? strains (USDA442 or USDA16) of Bradyrhizobium japonicum and uninoculated soybeans, plus a non-legume control [capsicum (Capsicum annuum L.)] were grown in the field at Ayr, North Queensland, Australia. The objectives were to examine (1) relationships between N₂ fixation and H₂ emission, and (2) the influence H₂-induced changes in soil might have during the legume phase and/or on the performance of a following crop. Strains CB1809 and USDA442 were highly effective in N₂ fixation (“good” fixers); USDA16 was partly effective (“poor” fixer). The soil had a large but non-uniformly distributed naturalised population of B. japonicum and most uninoculated control plants formed nodules that fixed some N₂. These naturalised strains were classified as “poor fixers” of N₂ and were Hup⁺. H₂ emissions from nodules were assessed for all treatments when the soybean crop was 62 days old. Other parameters of symbiotic N₂ fixation and plant productivity were measured when the crop was 62 and 96 days old and at crop maturity. Immediately after final harvest, the land was sown to a crop of maize (Zea mays L.) in order to determine the consequences of H₂ emission from the soybean crop on maize growth. It was estimated that soybeans inoculated with USDA442, the highly effective Hup^– strain of B. japonicum, fixed 117 kg shoot N/ha (or about 195 kg total N/ha if the fixed N associated with roots and nodules was taken into account), and contributed about 215,000 l H₂ gas per hectare to the ecosystem over the life of the crop. The volume of H₂ evolved from soybeans nodulated by the Hup⁺ strain CB1809 was only 6% of that emitted by the USDA442 treatment, but there was no indication that soybean inoculated with USDA442 benefited from the additional H₂ input. The shoot biomass, grain yield, and amounts of N fixed (105 kg shoot N/ha, 175 kg total N/ha) by the CB1809 treatment were little less than for USDA442 plants. Three days after the soybean crop was harvested, the plots were over-sown with maize along the same row lines in which the soybeans had grown. This procedure exposed the maize roots to whatever influence soybean H₂ emission might have had on the soil and/or the soil microflora immediately surrounding soybean nodules. The evidence for a positive effect of soybean H₂ emission on maize production was equivocal. While the consistent differences between those pre-treatments that emitted H₂ and those that did not indicated a trend, only one difference (out of the 12 parameters of maize productivity that were measured) was statistically significant at P?<?0.05. The findings need substantiation by further investigation.     

Development and Partial Characterization of Nearly Isogenic Pea Lines (Pisum sativum L.) that Alter Uptake Hydrogenase Activity in Symbiotic Rhizobium

Some Rhizobium bacteria have H₂-uptake (Hup) systems that oxidize H₂ evolved from nitrogenase in leguminous root nodules. Pea (Pisum sativum L.) cultivars `JI1205' and `Alaska' produce high Hup (Hup⁺⁺) and moderate Hup (Hup⁺) phenotypes, respectively, in Rhizobium leguminosarum 128C53. The physiological significance and biochemical basis of this host plant genetic effect are unknown. The purpose of this investigation was to advance basic Hup studies by developing nearly isogenic lines of peas that alter Hup phenotypes in R. leguminosarum strains containing hup genes. Eight pairs of nearly isogenic pea lines that produce Hup⁺⁺ and Hup⁺ phenotypes in R. leguminosarum 128C53 were identified in 173 F₂-derived F₆ families produced from crosses between JI1205 and Alaska. Tests with the pea isolines and three strains of hup-containing R. leguminosarum showed that the isolines altered Hup activity significantly (P ≤ 0.05) in 19 of 24 symbiotic combinations. Analyses of Hup phenotypes in F₆ families, the F₁ population, and two backcrosses suggested involvement of a single genetic locus. Three of the eight pairs of isolines were identified as being suitable for physiological studies, because the two lines in each pair showed similar growth, N assimilation, and flowering traits under nonsymbiotic conditions. Tests of those lines under N₂-dependent conditions with isogenic Hup⁺ and negligible Hup (Hup⁻) mutants of R. leguminosarum 128C53 showed that, in symbioses with Hup⁺ rhizobia, two out of three Hup⁺⁺ pea lines decreased N₂ fixation relative to Hup⁺ peas. In one of those cases, however, the Hup⁺⁺ plant line also decreased fixation by Hup⁻ rhizobia. When results were averaged across all rhizobia tested, Hup⁺ pea isolines had 8.2% higher dry weight (P ≤ 0.05) and fixed 12.6% more N₂ (P ≤ 0.05) than Hup⁺⁺ isolines. Pea lines described here may help identify host plant factors that influence rhizobial Hup activity and should assist in clarifying how Hup systems influence other physiological processes.     

Distribution of Hydrogen-Metabolizing Bacteria in Alfalfa Field Soil

H₂ evolved by alfalfa root nodules during the process of N₂ fixation may be an important factor influencing the distribution of soil bacteria. To test this hypothesis under field conditions, over 700 bacterial isolates were obtained from fallow soil or from the 3-mm layer of soil surrounding alfalfa (Medicago sativa L.) root nodules, alfalfa roots, or bindweed (Convolvulus arvensis L.) roots. Bacteria were isolated under either aerobic or microaerophilic conditions and were tested for their capacity to metabolize H₂. Isolates showing net H₂ uptake and ³H₂ incorporation activity under laboratory conditions were assigned a Hup⁺ phenotype, whereas organisms with significant H₂ output capacity were designated as a Hout⁺ phenotype. Under aerobic isolation conditions two Hup⁺ isolates were obtained, whereas under microaerophilic conditions five Hup⁺ and two Hout⁺ isolates were found. The nine isolates differed on the basis of 24 standard bacteriological characteristics or fatty acid composition. Five of the nine organisms were isolated from soil around root nodules, whereas the other four were found distributed among the other three soil environments. On the basis of the microaerophilic isolations, 4.8% of the total procaryotic isolates from soil around root nodules were capable of oxidizing H₂, and 1.2% could produce H₂. Two of the Hup⁺ isolates were identified as Rhizobium meliloti by root nodulation tests, but the fact that none of the isolates reduced C₂H₂ under the assay conditions suggested that the H₂ metabolism traits were associated with various hydrogenase systems rather than with nitrogenase activity. Results from this study support the concept that H₂ evolution by alfalfa root nodules has a significant effect on the surrounding microenvironment and influences the number and diversity of bacteria occupying that region.     

Host specificity of Rhizobium strains isolated from nitrogen-fixing trees and nitrogenase activities of strain GRH2 in symbiosis with Prosopis chilensis

Host plant specificity was examined in symbiosis between Rhizobium strains isolated from legume-tree root nodules and herbaceous or woody legumes from which they were isolated. Strain GRH2 isolated from Acacia cyanophylla formed effective nodules on Acacia, Prosopis and Medicago sativa as well. Nitrogenase activity, measured as acetylene reduction, of strain GRH2 in symbiosis with Prosopis chilensis was the highest (P 0.05) among the tropical legumes studied and was similar to those found for other associations involving herbaceous legumes. Relative efficiency of nitrogenase varied from 0.3 to near 1 during the light time of the photoperiod. However no hydrogen uptake activity was detected by the amperometric method used. Rhizobium strains GRH3, GRH5 and GRH9 isolated from A. melanoxylon, P. chilensis and Sophora microphylla, respectively, also showed a very low host-range specificity. All isolates were infective and effective on at least one of the herbaceous legumes tested. These data demonstrate the lack of specificity of Rhizobium strains isolated from nitrogen-fixing tree root nodules and that these strains can form effective nodules on herbaceous legumes.     

Economy of Photosynthate Use in Nitrogen-fixing Legume Nodules: Observations on Two Contrasting Symbioses

The economy of C use by root nodules was examined in two symbioses, Vigna unguiculata (L.) Walp. (cv. Caloona):Rhizobium CB756 and Lupinus albus L. (cv. Ultra):Rhizobium WU425 over a 2-week period in early vegetative growth. Plants were grown in minus N water culture with cuvettes attached to the nodulated zone of their primary roots for collection of evolved CO₂ and H₂. Increments in total plant N and in C and N of nodules, and C:N weight ratios of xylem and phloem exudates were studied by periodic sampling from the plant populations. Itemized budgets were constructed for the partitioning and utilization of C in the two species. For each milligram N fixed and assimilated by the cowpea association, 1.54 ± 0.26 (standard error) milligrams C as CO₂ and negligible H₂ were evolved and 3.11 milligrams of translocated C utilized by the nodules. Comparable values for nodules of the lupin association were 3.64 ± 0.28 milligrams C as CO₂, 0.22 ± 0.05 milligrams H₂, and 6.58 milligrams C. More efficient use of C by cowpea nodules was due to a lesser requirement of C for synthesis of exported N compounds, a smaller allocation of C to nodule dry matter, and a lower evolution of CO₂. The activity of phosphoenolpyruvate carboxylase in nodule extracts and the rate of ¹⁴CO₂ fixation by detached nodules were greater for the cowpea symbiosis (0.56 ± 0.06 and 0.22 milligrams C as CO₂ fixed per gram fresh weight per hour, respectively) than for the lupin 0.06 ± 0.02 and 0.01 milligrams C as CO₂ fixed per gram fresh weight per hour. The significance of the data was discussed in relation to current information on theoretical costs of nitrogenase functioning and associated nodule processes.     

The Relationship between H(2) Evolution and Acetylene Reduction in Pisum sativum-Rhizobium leguminosarum Symbioses Differing in Uptake Hydrogenase Activity

Peas (Pisum sativum L.) were inoculated with strains of Rhizobium leguminosarum having different levels of uptake hydrogenase (Hup) activity and were grown in sterile Leonard jars under controlled conditions. Rates of H₂ evolution and acetylene reduction were determined for intact nodulated roots at intervals after the onset of darkness or after removal of the shoots. Hup activity was estimated using treatment plants or equivalent plants from the growth chamber, by measuring the uptake of H₂ or ³H₂ in the presence of acetylene. In all cases, the rate of H₂ evolution was a continuous function of the rate of acetylene reduction. In symbioses with no demonstrable Hup activity, H₂ evolution increased in direct proportion to acetylene reduction and the slopes were similar with the Hup⁻ strains NA502 and 128C79. Hup activity was similar in strains 128C30 and 128C52 but significantly lower in strain 128C54. With these strains, the slopes of the H₂ evolution versus acetylene reduction curves initially increased with acetylene reduction, but became constant and similar to those for the Hup⁻ strains at high rates of acetylene reduction. On these parallel portions of the curves, the decreases in H₂ evolution by Hup⁺ strains were similar in magnitude to their H₂-saturated rates of Hup activity. The curvilinear relationship between H₂ evolution and acetylene reduction for a representative Hup⁺ strain (128C52) was the same, regardless of the experimental conditions used to vary the nitrogenase activity.     

Persistence and recovery of introduced Rhizobium ten years after inoculation on Leucaena leucocephala grown on an Alfisol in southwestern Nigeria

Establishment of Leucaena leucocephala was poor at Ibadan (Transition forest-savanna zone) and Fashola (savanna zone, 70 km north of Ibadan) in southwestern Nigeria as a result of low soil fertility and the presence of only a few native rhizobia capable of nodulating it. Inoculation with L. leucocephala at these two locations in 1982 resulted in striking responses with Rhizobium strains IRc 1045 and IRc 1050 isolated from L. leucocephala grown in Nigeria. The persistence of inoculated effective Rhizobium strains after inoculation is desirable since it removes the need for reinoculation. Because of the perennial nature of L. leucocephala and its use in long-term alley farming experiments, we examined the persistence of inoculated rhizobial strains after inoculation, and their ability to sustain N₂-fixation and biomass production at Ibadan. In 1992, ten years after Rhizobium introduction, uninoculated, L. leucocephala fixed about 150 kg N ha^-1 yr^-1 or about 41% of total plant N compared to 180 kg N ha^-1 yr^-1 or 43% measured in 1982. Serological typing of the nodules using the Enzyme-Linked-Immunosorbent Assay (ELISA) and intrinsic resistance to the streptomycin test revealed that most of the nodules (96%) formed on L. leucocephala in 1992 were by Rhizobium strains IRc 1045 and IRc 1050, which were inoculated in 1982. Nodules were absent on uninoculated L. leucocephala grown on the adjacent field with no history of L. leucocephala cultivation. We conclude that the N₂ fixed by Rhizobium strains IRc 1045 and IRc 1050 persisted for many years in the absence of L. leucocephala and sustained effectively fixed N₂ which growth and yield of L. leucocephala after several years, thus encouraging a possible low-input alley farming system by smallholder farmers in Nigeria.     

Hydrogen accumulation by H2-uptake negative strains of Rhizobium

Summary Hydrogen evolution from root nodules has been reported to decrease the efficiency of the nitrogen fixing system. Mutants ofRhizobium meliloti andRhizobium leguminosarum were selected which were deficient in H₂-uptake capacity (Hup^–). The relative efficiency of the nitrogen fixation for both species assessed with C₂H₂ reduction was 0.66.The hydrogen production was monitored using a simple root incubation method. As such, hydrogen production up to 3.83 and 15.57 ml.day^–1.g^–1 plant dry weight were recorded forPisum sativum — Rhizobium leguminosarum 4.20 Hup^– andMedicago sativa — Rhizobium meliloti 1.5 Hup^– respectively. In a closed container (250 ml), hydrogen concentrations up to 20% (v/v) could be reached in the root phase ofMedicago sativa in a time period of 320 hours.     

Selection of symbiotically energy efficient strains of Rhizobium japonicum by their ability to induce a H2-uptake hydrogenase in the free-living state

The capacity of inducing a H₂-uptake hydrogenase in free-living cultures was examined in 21 strains of Rhizobium japonicum. Four strains were found to take up H₂ at rapid rates after 3 days of growth on agar slants inside sealed vials provided with an atmosphere of 5% H₂ in air. Soybean nodules from these strains lost little or no H₂ in air and their bacteroids oxidized H₂ at rates that were similar to those observed in free-living cultures. In contrast, three randomly chosen strains of R. japonicum that showed no H₂-uptake capacity in free-living state produced nodules which lost large amounts of H₂ and the corresponding bacteroids had no hydrogenase activity. A screening procedure is described for the selection of Rhizobium strains producing high energy-efficient nodules based on a test of their ability to induce a H₂-uptake hydrogenase in asymbiotic conditions.     

Production and composition of extracellular polysaccharide synthesized by a Rhizobium isolate of Vigna mungo (L.) Hepper

An extracellular polysaccharide (EPS) was produced by a Rhizobium sp. isolated from the root nodules of Vigna mungo (L.) Hepper. Maximum EPS production (346 mg l⁻¹) was when the yeast extract basal medium was supplemented with mannitol (1%), biotin (1.5 mg l⁻¹) and asparagine (0.3%). Ribose (53%) and mannose (47%) were the principle monomers of the EPS. Chemical, chromatographic and spectroscopic analysis showed that this polymer, which has Man₄Rib₁ as an oligomeric subunit, has an apparent molecular mass of 750 kDa.     

Conservation in Soil of H2 Liberated from N2 Fixation by Hup- Nodules

Pigeon peas (Cajanus cajan) were grown in large soil columns (90-cm length by 30-cm diameter) and inoculated with four different strains of cowpea rhizobia, which varied with respect to hydrogen uptake activity (Hup). Despite the profuse liberation of H₂ from Hup^- nodules in vitro, H₂ gas was not detected in any of the soil columns. When H₂ was injected into the columns, the rates of consumption were highest in the treatments (including control) containing Hup^- nodules (218 and 177 nmol · h⁻¹ · cm⁻²) and lowest in the Hup⁺ treatments (158, 92, and 64 nmoles · h⁻¹ · cm⁻²). In situ H₂ uptake rates in small soil cores at fixed distances from the nodules decreased exponentially with distance from the nodule (R² = 0.99). This decrease in H₂ consumption was associated with a similar decrease in numbers of H₂-oxidizing chemolithotrophic bacteria as determined by the most-probable-number method. On the basis of two equations derived separately upon diffusive theory (Fix's Law) and kinetic theory (Michaelis-Menten), the empirically derived rate constants and coefficients indicated that all of the H₂ emitted from Hup^- nodules would be consumed by H₂-oxidizing bacteria within a 3- to 4.5-cm radius of the nodule surface. It is concluded that H₂ is not lost from the soil-plant ecosystem during N₂ fixation in C. cajan but is conserved by H₂-oxidizing bacteria.     

<Emphasis Type="Italic">Rhizobium helanshanense</Emphasis> sp. nov., a bacterium that nodulates <Emphasis Type="Italic">Sphaerophysa salsula</Emphasis> (Pall.) DC. in China

Studying rhizobia in the root nodules of Sphaerophysa salsula (Pall.) DC in the northwest of China, we obtained five strains classified as genus Rhizobium on the basis of their 16S rRNA gene sequences. The sequence similarity of strain CCNWQTX14^T with the most related species was 99.0%. Further phylogenetic analysis of housekeeping genes (recA and atpD) suggested the five strains comprised a novel lineage within Rhizobium. The nifH and nodD gene sequences of CCNWQTX14^T were phylogenetically closely related with those of Sinorhizobium kummerowiae and R. sphaerophysae, respectively. The five strains isolated from different places were also distinct from related Rhizobium species using ERIC fingerprint profiles. The DNA–DNA hybridization value was 41.8% between CCNWQTX14^T and Rhizobium sphaerophysae CCNWGS0238^T. Our novel strains were only able to form effective nodules on its original host Sphaerophysa salsula. Our data showed that the five Rhizobium strains formed a unique genomic species, for which a novel species Rhizobium helanshanense sp. nov. is proposed. The type strain is CCNWQTX14^T (=ACCC 16237^T =HAMBI 3083^T).     

Enrichment for Hydrogen-Oxidizing Acinetobacter spp. in the Rhizosphere of Hydrogen-Evolving Soybean Root Nodules

Field soybean plants were inoculated with Hup⁺ wild-type or H₂ uptake-negative (Hup⁻) mutants of Bradyrhizobium japonicum. For two consecutive summers we found an enrichment for acinetobacters associated with the surfaces of the H₂-evolving nodules. Soybean root nodules that evolved H₂ had up to 12 times more Acinetobacter spp. bacteria associated with their surfaces than did nodules incapable of evolving H₂. All of the newly isolated strains identified as Acinetobacter obtained from the surfaces of root nodules, as well as known established Acinetobacter strains, were capable of oxidizing H₂, a property not previously described for this alkane-degrading soil bacterium.